The manufacture of photovoltaic solar cells involves provision of semiconductor substrates in the form of sheets or wafers having a shallow p-n junction adjacent one surface thereof (commonly called the "front surface"). Such substrates, which may include an insulating anti-reflection ("AR") coating on their front surfaces, are commonly referred to as "solar cell blanks". The anti-reflection coating is transparent to solar radiation. In the case of silicon solar cells, the AR coating is often made of silicon nitride or an oxide of silicon or titanium.
A typical solar cell blank may take the form of a rectangular EFG-grown polycrystalline silicon substrate of p-type conductivity having a thickness in the range of 0.008 to 0.018 inches and a p-n junction located about 0.5 microns from its front surface, and also having a silicon nitride coating about 800 Angstroms thick covering its front surface. Equivalent solar cell blanks also are well known, e.g. those comprising single crystal silicon substrates and cast polycrystalline silicon substrates.
The solar cell blanks are converted to finished solar cells by providing them with electrical contacts (sometimes referred to as "electrodes") on both the front and rear sides of the semiconductor substrate, so as to permit recovery of an electrical current from the cells when they are exposed to solar radiation. These contacts are typically made of aluminum, silver, nickel or other metal or metal alloy. A common arrangement is to provide silicon solar cells with rear contacts made of aluminum and front contacts made of silver.
The contact on the front surface of the cell is generally in the form of a grid, comprising an array of narrow fingers and at least one elongate bus (also hereinafter called a "bus bar") that intersects the fingers. The width and number of the fingers and busses are selected so that the area of the front surface exposed to solar radiation is maximized. Further to improve the conversion efficiency of the cell, an AR coating as described overlies and is bonded to those areas of the front surface of the cell that are not covered by the front contact.
The rear contact may cover the entire rear surface of the solar cell blank, but more commonly it is formed so as to terminate close to but short of the edges of the blank. Aluminum is preferred for the rear contact for cost and other reasons. However, the exposed surface of an aluminum contact tends to oxidize in air, making it difficult to solder a wire lead to the contact. Therefore, to facilitate soldering, it has been found useful additionally to provide apertures in the aluminum coating, with silver soldering pads being formed in those apertures so as to slightly overlap the adjacent aluminum layer. The silver pads form ohmic bonds with the underlying substrate and also low resistance electrical connections with the aluminum contact, and are used as sites for making soldered connections to the rear contact. The silver soldering pads are considered to be an integral part of the rear contact. Such a contact arrangement is disclosed in PCT International Publication No. WO 92/02952, based on U.S. patent application Ser. No. 07/561,101, filed Sep. 1, 1990 by Frank Bottari et al for "Method Of Applying Metallized Contacts To A Solar Cell". An alternative but similar back contact arrangement wherein the aluminum coating has apertures filled with silver soldering pads involves having the aluminum overlap the edges of the silver soldering pads.
The grid-shaped contact and the AR coating on the front surface may be formed in various ways, as exemplified by U.S. Pat. Nos. 4,451969, 4,609,565, 4,751,191, 5,010,040, 5,074,920, British Patent No. 2,215,129, and PCT International Application WO 89/12321, published 14 Dec. 1989.
Regardless of how the front grid contact and the AR coating are formed, at least a portion of each bus of the front contact is not covered with the AR coating, so as to permit making a soldered connection to that contact.
Photovoltaic solar cells (e.g., silicon solar cells) are typically small in size, e.g., 2-4 inches on a side, with the result that their power output also is small. Hence, industry practice is to interconnect a plurality of cells so as to form a physically integrated module with a correspondingly greater power output, and then several such modules are in turn assembled and interconnected to form a solar panel. Several solar panels may be connected together to form a larger array. The cells in a module are electrically connected in parallel and/or in series, and two or more modules in a panel may be connected in series or in parallel, depending on the voltage and current output that is desired from the panel.
A common practice is to use copper wire, preferably in the form of strips of flat copper ribbon, to interconnect a plurality of cells in a module, with each ribbon being soldered to the front or back contact of a particular cell by means of a suitable solder paste, e.g., a solder paste as described in U.S. Pat. No. 5,074,920. The solder paste is deposited onto the contacts at ambient temperatures, preferably as discrete small daubs, and then with a copper ribbon contacting it, each daub is heated just enough, preferably by exposure to hot air, to drive off the fluxing agent and cause the metal components of the solder to fuse to the copper ribbon and the underlying solar cell contact.